Laccases (E.C. 1.10.3.2; benzenediol oxygen oxidoreductases) couple the four-electron reduction of dioxygen to water to four one-electron oxidations of a reducing substrate. The three-dimensional structure of the 'blue' multi-copper oxidase laccase from the fungus Coprinus cinereus at 1.68 A reveals the structural basis for isoforms of the type 2 Cu-depleted species.